We will study the proteins of the platelet plasma membrane to characterize the molecular basis of platelet function. Specific membrane proteins will be identified which are altered during platelet storage in vitro or circulation in vivo. Although platelet adhesion is the primary event in both hemostasis and thrombosis, little is known about the specific proteins involved in platelet contact. The ability to solubilize and separate membrane proteins in sodium dodecyl sulfate-polyacrylamide gel electrophoresis has allowed the characterization of the individual proteins. Nonpenetrating membrane probes have allowed further definition of membrane organization by labeling exposed surface proteins. We have developed a low molecular weight, nonpenetrating radioactive label, 125I-diazotized diiodosulfanilic acid (DD125ISA), and we have demonstrated that it specifically binds to the glycoproteins on the outer surface of the plasma membrane. Also we have developed methods to quantitatively evaluate platelet membrane proteins. We will use these methods to determine the changes of membrane proteins during circulation and storage. We hope that our studies will further characterize the platelet surface and lead to better understanding of the platelet's role in hemostasis and thrombosis.